1. Technical Field
The present invention relates to manufacturing methods for ferroelectric memory devices having ferroelectric capacitors.
2. Related Art
A ferroelectric memory device (FeRAM) is a nonvolatile memory that uses spontaneous polarization of ferroelectric material and is capable of low voltage and high speed operations. In such a ferroelectric memory device, its memory cell can be formed from one transistor and one capacitor (1T/1C), such that integration to the level of DRAM is possible. Accordingly, ferroelectric memory devices are highly expected as large capacity nonvolatile memories. As the ferroelectric material, perovskite type oxides such as lead zirconate titanate (Pb (Zi, Ti) O3: PZT), and bismuth layered compounds such as strontium bismuth tantalate (SrBi2TaO9: SBT) can be enumerated.
In order to make the aforementioned ferroelectric material exhibit its maximum ferroelectric characteristic, its crystal orientation is extremely important. For example, when PZT is used as the ferroelectric material, a predominant orientation exists depending on its crystal system. Generally, when PZT is used in memory devices, titanium-rich compositions that contain a greater amount of Ti compared to Zr is used in order to obtain greater spontaneous polarization. In such a composition range, PZT belongs to a tetragonal system, and its spontaneous polarization axis aligns with the c-axis. In this case, ideally, the maximum polarization can be obtained by orienting it in the c-axis, which is in effect very difficult, and a-axis oriented components perpendicular to the c-axis concurrently exist. Because the a-axis orientated components do not contribute to polarization inversion, the ferroelectric characteristic may be degraded.
In this respect, it has been conceived to orient the a-axis in a direction offset at a predetermined angle from the substrate normal line by making the crystal orientation of PZT to (111). As a result, the polarization axis has a component in the substrate normal line direction, and thus can contribute to polarization inversion. On the other hand, the c-axis oriented component concurrently has its polarization axis oriented to a predetermined offset angle with respect to the substrate normal line direction, such that a certain amount of loss occurs in the amount of surface charge induced by polarization inversion. However, the entire crystal components can be made to contribute to polarization inversion, such that the charge retrieving efficiency significantly excels, compared to the case of the c-axis orientation.
In order to make PZT to have a (111) orientation, the crystal orientation of the lower electrode on which the PZT film is formed is important. As the material composing the lower electrode, a noble metal such as Ir (iridium) may be used in consideration of its thermal and chemical stability. However, for obtaining the (111)-oriented PZT film, the r film needs to be oriented in <111>. However, because the self-orientation properties of the Ir film are weak, the Ir film needs to be formed on a base layer whose crystal orientation is in <111>. However, even when the Ir film is oriented to (111), it has been difficult to make the crystal orientation of the PZT film in <111>.
In view of the above, there has been proposed a method in which an IrOx film that is an oxide of Ir is formed by applying an oxidation treatment to a surface of an Ir film that forms a lower electrode, and PZT is formed on the surface of the IrOx film by a MOCVD (Metal Organic Chemical Vapor Deposition) method, whereby the PZT has a (111) orientation. For example, Japanese Unexamined Patent Application, First Publication No. 2003-324101 is an example of related art.
However, the aforementioned manufacturing method for a ferroelectric memory device still entails the following problems. Because the oxide film is formed by thermal oxidation, it is difficult to control the film quality of the oxide film. For example, when a uniform oxide film is not formed and localized excessive oxidation occurs, hillocks that are small protuberances may be generated in the lower electrode due to volume expansions caused by the localized excessive oxidation. Also, because the oxide film is formed at the lower electrode by thermal oxidation advancing from its surface layer portion toward its inner layer portion, oxidation is inhibited by the oxide film formed on the surface, and it would become difficult for the oxidation to advance to an inner layer portion of the lower electrode. In other words, the oxide film formed on the surface shows an oxidation barrier properties and oxidation becomes difficult to advance to an inner layer portion of the lower electrode, such that it is difficult to increase the oxygen concentration of the lower electrode. As an oxide film having the appropriate oxygen concentration and appropriate film thickness cannot be obtained, a problem occurs in that the crystal orientation of a ferroelectric layer formed thereon can not be controlled (is not stable). Furthermore, because the oxidation treatment in which the lower electrode is exposed to an oxygen atmosphere at high temperatures is conducted, other elements may be thermally impacted.